Pressure sensors are applied in pressure measuring technology for metrological registering of pressures. In such case, ceramic pressure measurement cells have an advantage that, due to the chemical and mechanical durability of ceramic, they can be exposed via an opening in the pressure sensor directly to a medium under a pressure to be registered metrologically. To this end, the pressure measurement cells are regularly clamped in the pressure sensor with interpositioning of a seal outwardly sealing an interior of the pressure sensor and supplied with the pressure to be registered via an opening of the pressure sensor.
Frequently applied as seals are elastomers. There are, however, applications, in the case of which elastomers cannot be applied, e.g. because they cannot withstand the arising chemical and thermal conditions. Metal seals often do not work, because of the required clamping forces.
In such cases, seals, especially flat seals, of chemically and/or thermally more resistant thermoplastics, such as e.g. polytetrafluoroethylene (PTFE), are used.
It is, however, difficult, to clamp the seals, on the one hand, sufficiently tightly, and, on the other hand, without degrading the accuracy of measurement of the pressure sensor to be sealed.
Described in DE 196 28 551 A1 is a pressure sensor with, arranged in a housing, a pressure measuring cell, in the case of which a PTFE seal is clamped between a resiliently formed sealing spring ridge of the housing of the pressure sensor and the pressure measuring cell, wherein the sealing spring ridge has a groove for accommodating the PTFE seal. The comparatively substantial seal has a thickness of 0.8 mm and is chambered, which is to prevent that the thermoplastic sealing material is squeezed too strongly by the compression between the sealing surfaces. In this case, however, comparatively large clamping forces are required, in order to achieve a reliable sealing. Large clamping forces can, however, degrade the accuracy of measurement of the pressure sensor, at least at low pressures, e.g. in the case of pressures below 1 bar. Moreover, hysteresis phenomena are to be expected in the case of the described sealing arrangement, since the elastic housing-side sealing surfaces must lead to shearing in the thermoplastic sealing material.
Both in DE 103 34 854 A1, as well as also in DE 10 2004 057 967 A1, a pressure sensor is described,                which has a ceramic pressure measuring cell clamped in the pressure sensor with interpositioning of a seal outwardly sealing an interior of the pressure sensor and loadable via an opening of the pressure sensor with a pressure to be measured, and        whose seal comprises a film of a thermoplastic material, especially polytetrafluoroethylene (PTFE), clamped (in an axial direction extending perpendicularly to planes of the sealing surfaces) between a form-retaining, planar sealing surface of the pressure measuring cell and a form-retaining sealing surface of a counterbody outwardly surrounding the opening.        
Due to the clamping of the film formed in such case as a flat seal between two form-retaining sealing surfaces, a reliable sealing can be achieved with significantly smaller clamping forces.
In the case of both pressure sensors, the counterbody is embodied as a decoupling ring clamped between the pressure measuring cell and a shoulder of the housing surrounding the opening. The decoupling ring outwardly surrounds the opening, and protects the pressure measuring cell especially against stresses acting in the radial direction, thus perpendicular to the surface normal to the sealing surfaces. To this end, the decoupling ring is preferably composed of the ceramic of the pressure measuring cell.
In the case of the pressure sensor described in DE 103 34 854 A1, the counterbody is embodied as a ceramic ring, which has, protruding in the axial direction, formed through a number of annular, concentric ridges, structures, which, on the one hand, press the flat seal in, and, on the other hand, bring about in their intermediate spaces a kind of chambering for the sealing material.
In the case of the pressure sensor described in DE 10 2004 057 967 A1, the decoupling ring has in the axial direction a protrusion, whose planar end forms the sealing surface of the counterbody. In this way, the main closure of the flat seal clamped between the form-retaining, planar sealing surfaces is laterally exposed. The exposed clamping means that a movement or deformation the flat seal parallel to the planar sealing surfaces is not constrained by additional design elements, for instance, by a chambering.
This form of embodiment offers the advantage that defined compression of the seal can be achieved.
In the case of both pressure sensors, however, reproducible clamping of the flat seals proves to be difficult.
For overcoming the aforementioned disadvantages, the unpublished German patent application DE 10 2013 111 910.7 of the applicant, filed Oct. 29, 2013, describes a seal applied as a coating on a planar sealing surface of the counterbody. The coating includes a first layer serving as bonding agent applied directly on the sealing surface of the counterbody, e.g. a first layer of perfluoroalkoxy-polymer (PFA), on which a second layer of a thermoplastic material, e.g. polytetrafluoroethylene (PTFE), is applied. Both layers border on the opening of the pressure sensor, so that they are exposed in measurement operation to the medium supplied via the opening for metrologically registering its pressure.
There are, however, applications, in the case of which materials usable as bonding agent, especially due to their chemical properties, are not permitted to be directly contacted by the medium.
Moreover, the production of high value coatings, especially homogeneous and as pore free as possible coatings, is complex in comparison to the use of prefabricated, flat seals.